A method and a device for mapping the total refraction non-homogeneity of an eye are set forth in prior co-pending PCT Application No. PCT/US99/23327 that includes directing into the eye a narrow laser beam, its axis being parallel to the visual axis of the eye under investigation, scanning the beam over the eye aperture, receiving a portion of light scattered by the retina, analyzing the position of the laser spot projected on the retina, and reconstructing from the data a map of the total refraction of the eye. A discussion of the details of the total refraction non-homogeneity determination have been incorporated herein and set forth below.
In many applications, information on the contribution of other refractive components of the eye may be helpful or necessary, as, for example, for subsequent corrective surgery.
For the purpose of measuring the surface shape of a cornea, a method is known of projecting a regular structure or regular patterns, such as a pattern of concentric disks onto the cornea, analyzing the reflected light and reconstructing from the analyzed data the shape and therefore the refraction distribution caused by the cornea. It has been discovered by Applicants that marrying the techniques for analyzing retina-scattered light and for analyzing cornea reflected light may give very useful information on the contribution to the total eye refraction of such other refractive components of the cornea, and/or the eye lens, that has not heretofore been successfully accomplished.
Measuring devices are known, for the study of the refraction component of the optical system of the eye, which depend on spatial pupil coordinates. These include M. S. Smimov's Smirnov's device for measuring the wave aberration [1], Van den Brink's device for measuring the transverse aberration [2], N. M. Sergienko's device for measuring the physiological astigmatism [3], and a spatially resolved refractometer [4]. The above devices, based on Scheiner's principle, involve point-by-point investigation over utilizing a number of optical techniques. However, in using all such devices the direct participation of the patient is needed in the preliminary aligning of the eye and in the aberration measurements.
Major disadvantages of the above measuring devices are their low accuracy and productivity, a prolonged measurement process resulting in the patient's fatigue, variations in accommodation, and eye movements while taking measurements, thereby increasing the aberration measurement errors.
More advanced measuring devices are known, which do not require the patient to act as a link in the “measurement chain”. These include a device for measuring the aberration by the Foucault's knife method [5], a device for measuring the wave aberration using Hartmann-Shack sensors [6-8], including measurements that incorporate adaptive optics completely compensating the wave aberration [9].
A common disadvantage of the measuring devices with a Hartmann-Shack sensor is the fixed field of view of the raster photoelectric analyzer of transverse aberrations due to the mechanically rigid construction of the lens raster and the invariable mutual spatial arrangement of the photosensitive elements of the charged coupled device or CCD camera. This results in a fixed configuration of grid sites at the pupil plane in which aberrations are measured, with no flexibility of reconfiguring these grid sites for more detailed measurements in separate zones of the pupil depending on their aberration properties.
Other disadvantages of existing devices include: they do not incorporate means for providing an accurate reproducible “linkage” of the patient's eye to the spatial co-ordinates of the measuring device; they do not incorporate a means for adjusting the accommodation of the patient's eye that is necessary for studying the dependence of aberrations on the accommodation characteristics; they are not capable of taking measurements on a dilated pupil without using medicines.
Refraction can also be measured using a spatially resolved objective autorefractometer as disclosed in U.S. Pat. No. 5,258,791 [10]. This device provides spatially resolved refraction data using a closed measuring loop which includes a reference pattern and a measurement beam. In this device, an origin of coordinates of the detector coincides with the center of the fovea image and the detector functions as a zero-position sensor.
The spatially resolved objective autorefractometer disclosed in U.S. Pat. No. 5,258,791 preferably using laser ray tracing, has a number of substantial problems relating to performance in the following basic and auxiliary functions: preliminary alignment of the optical axis of the device relative to the visual axis of the eye; accommodation monitoring of the patient's eye; allocation of points within the pupil at which refraction is measured; and measurement of the angle of laser beam incidence into the patients eye. Respective to the above basic and auxiliary functions, the above drawbacks are inherent to the device disclosed in U.S. Pat. No. 5,258,791.
Preliminary alignment of the optical axis of prior art devices relative to the visual axis of the eye may be problematic for at least the following reasons: first, the visual axis of the eye is assumed to be the line passing through the geometric center of the pupil and the fovea. However, it is known that the geometric center of the pupil does not always coincide with the visual axis due to the misalignment of the pupil opening and the optical axis of the cornea and the crystalline lens. In addition, the pupil may not be symmetrical.
Second, in prior art devices in which alignment is dependant on fixation of the patient's gaze at a focal point, changes in the position of the point at which the patients gaze is fixed results in angular movement of the patient's eye which disturbs the previous alignment. Consequently, both points (on the pupil and on the retina) through which the center line passes do not have a definite location.
Third, the focal points in devices without ametropia compensation can clearly be observed only with an emmetropic or normal eye. When the patient's eye is ametropic, such devices will see a diffused laser beam spot whose width increases with the ametropy. It is obvious that under such conditions the gaze cannot be fixed accurately in a certain direction, which is another factor preventing an accurate alignment. Another drawback of prior art refractometers is that the fovea and the photosensitive surface of the photodetector are optically coupled by the lenses only in the emmetropic or normal eye. In the event of an ametropic eye, the decentering or defocusing of the fovea image on the above-mentioned surface of the photoelectric detector causes additional refraction measurement errors which are not compensated for. The present invention is designed to compensate for this.
Fourth, a sufficiently bright laser radiation may irritate the fovea to such a degree that the eye begins to narrow its pupil reflexly. Therefore, before performance of the eye centering procedure, medicines paralyzing the ciliar body muscles are likely to be required, which changes the refractive properties of the eye as compared with its normal natural state.
The need for accommodation monitoring of the patient's eye has not been satisfied in prior art devices. As a consequence, the patient's eye can be accommodating at any distance. It is known that the refractive properties of the eye depend on the accommodation distance. Because the accommodation is unknown to the operator, it is impossible to correlate the refraction map and the eye accommodation.
It has become apparent to the present applicants that a spatially resolved refractometer should preferably include a device for adjusting to the patient's eye accommodation.
Prior art devices using electromechanical actuators greatly reduce the possibility of ensuring a high-speed scanning of the pupil and the possibility of shortening the duration of the ocular refraction measurement process.
In prior art devices using a disc or movable aperture bearing planar surface to control laser targeting, the aperture occupies only a small portion of the zone in which the laser beam intersects the planar surface. Thus, only that portion of the laser beam which is equal to the ratio of the area of one refraction measurement zone on the pupil to the entire pupil area passes through the aperture. Such a vignetting of the laser beam results in an uneconomical use of laser radiation and should be considered a major drawback of such designs.
Drawbacks in the measurement of the angle of laser beam incidence at which it crosses the necessary measurement zone of the pupil and the center of the fovea are inherent to designs which do not provide a sufficiently high refraction measurement speed. In such designs, the time of measuring the refraction at 10 measurement points of the pupil is up to one minute. During this period the patient's eye can move up to 100 times and change its angular position due to natural tremor, “jumps” and drift.
Systematic instrument errors have plagued prior aberration refractometeis. Due to an irregular distribution of the light irradiance within the light spot on the retina, unequal photosensitivity across the surface photoelectric detector, time instability of the gain of preamplifiers connected to the photoelectric detector elements, and the presence of unsuppressed glares and background illumination the photodetector, the photodetector does not register a “zero” position of the spot on the fovea without systematic errors. Further, as a result of its own aberrations, the optical system providing for eye ray tracing contributes an angular aberration to the laser beam position. The present instrument incorporates structural elements which compensate for such errors and thus increase the refraction measurement accuracy.
What is needed is an improved electro-optical ray tracing aberration refractometer which makes it possible to achieve the following goals: flexibility of allocation of measuring points within the pupil and to pupil and improvement in the effectiveness of using lasers by reducing the vignetting of the laser beam at the aperture diaphragm; reduction in the duration of measuring refraction across the entire pupil to around 10-20 milliseconds; ensuring optical coupling of the photosensitive surface of the photodetector with the fovea even for ametropic eyes as well as for accommodation monitoring at any given distance; synchronously measuring both the total eye refraction aberrations and the component caused by cornea refraction characteristics maintaining incremental accuracy by interposing additional impingement points between impact points with wide variation in refraction characteristics; reduction in instrument errors when measuring aberration refraction; enhancement of the accuracy and definitiveness of instrument positioning relative to the patient's eye; the potential for automation of the positioning and controllability of the working distance between the patient's eye and the device components; and enablement of instrument positioning without medically dilating the pupil. The present invention provides the aforementioned solutions and innovations.
It has also been found that, due to noncoincidence of the points of primary information from the attempted combination of total eye refraction information and the surface shape of a cornea, and the followed approximation, any maps reconstructed from such cornea topography data and separately obtained total eye refraction aberration data would contain significant errors when reconstructing the differences. A workable device and method for synchronous mapping of the total refraction non-homogeneity of the eye and of the refraction components caused by the cornea would be desirable.